The present invention relates to a lead alloy foil for a laminated tape hereinafter referred to as "lead alloy foil" and a lead laminated tape using the foil, both designed for covering cables.
Generally, power cables insulated with rubber or plastics to withstand high voltages have the structure shown in FIG. 1. More specifically, such a cable comprises conductor 1, conductor shield 2 covering conductor 1, insulation layer 3 covering layer 2 and made of polyethylene, crosslinked polyethylene, ethylenepropylene rubber, or the like, insulation shield 4 covering insulation layer 3, metal shield layer 5 covering layer 4 and made of copper tape, copper wire, aluminum wire, or the like, and, if necessary, jacket 6 covering metal shield layer 5.
In a power cable of the structure described above, water infiltration may occur either along the conductor or from the outside to the inside of the cable, through the ends, splicing portions, and/or the jacket, during the manufacture, storage, installation or use of the cable. Water may penetrate from the conductor to the conductor shield and thence to the insulation layer. When AC voltage is applied to the cable insulation into which much water has infiltrated in this manner, fine defects called "water-trees" are formed in the insulation layer. These water-trees degrade the insulation characteristics of the insulation layer and may cause an electrical failure of the cable after a long use.
Accordingly, it is strongly demanded today that water should be prevented from infiltrating into high-voltage power cables insulated with rubber or plastics.
It has been proposed that a water impervious layer comprising a foil, such as lead foil and lead alloy foil having a thickness of several microns, should cover the insulation layer of the cable or be provided under the anti-corrosion plastic jacket of the cable. This is because the lead or lead alloy foil is very flexible and resistant to chemicals. Such lead or lead alloy foil is actually used in some of the power cables available at present.
More precisely, the lead foil or lead alloy foil, both hereinafter referred to as "lead foil", is used as a component of a laminated tape. The laminated tape comprises the lead foil and an electrically conductive or insulating, plastic film laminated on one surface of the foil, or two such plastic films laminated on both surfaces of the foil. The tape is formed around the conductor and/or core of a cable, by the process called "longitudinal application." The laminated tape is then thermally bonded to a layer covering the cable core or an anti-corrosion plastic jacket, thereby forming an water impervious layer.
This method of forming the impervious layer is advantageous over the conventional method wherein lead is extruded over the cable core; it can form a lead foil thinner than in the conventional method, and hence helps to lighten power cables.
However, several problems arise from the use of the laminated tape having a foil made of pure lead or lead alloy consisting of 5 wt. % (weight percent) of tin, 2 wt. % of antimony, and the balance being lead and having a thickness of 50 to 100 microns. More precisely, the foil of lead or lead alloy is disadvantageous in the following respects:
(1) Its mechanical strength is insufficient, making it difficult to form the water impervious layer during the manufacture.
(2) It has inadequate fatigue characteristics, and is very likely to break when bent once or repeatedly. In short, it does not have an enough fatigue strength.
(3) It will have pinholes or cracks when formed to have a thickness of 50 microns or less. In other words, it cannot be made 50 microns thick or thinner.
(4) It encounters surface corrosion shortly after manufacture, reducing the bonding strength between it and plastic films.